1. Field of the Invention
This invention relates to electronic radiography and more particularly to automatic exposure control systems and methods.
2. Description of Related Art
Electronic radiographic imaging, also commonly referred to as direct radiographic imaging, using imaging panels comprising a two dimensional array of minute sensors to capture a radiation generated image is well known in the art. Information representing an image is captured, usually as a charge distribution stored in a plurality of charge storage capacitors, in individual sensors arrayed in a two dimensional matrix. Readout of the stored charges provides an electrical signal which may be used to display the captured charges as a radiographic image on a display device, such as a cathode ray tube.
In a typical arrangement, an X-ray radiation source is used to provide a radiation beam. The radiation beam is aimed at an imaging panel at a known and usually fixed position, spaced from the source. A patient is positioned in front of the imaging panel between the imaging panel and the radiation source so that the radiation beam passes through the patient's body before impinging thereon.
The patient's body absorbs and scatters a certain amount of the radiation modulating the radiation intensity exiting the body and impinging on the imaging panel. The modulated radiation generates charges in the array of the sensors forming the panel in proportion to its intensity and the time during which the radiation impinges on the panel.
The duration of radiation emission is critical in generating an image having optimum diagnostic characteristics. If the time is too long the results will be unnecessarily high radiation dose to the patient and possible saturation of the detector. If the time is too short, the results will be lack of detail in certain areas, as well as excessive quantum noise (mottle) in the image. In some cases the radiographic examination may need to be repeated, exposing the patient to additional radiation, as well as delaying the evaluation of the examination results and increasing the cost of the procedure.
The solution to this problem is an automatic exposure control system in which the radiation intensity through the patient is monitored and the exposure terminated after a certain time resulting in a desired exposure of the imaging panel to the impinging radiation. Such systems typically use one or more radiation sensors placed in front of, or behind, the panel to measure the amount of incident radiation impinging on the sensor generating an electrical output proportional to the radiation intensity. This output is integrated over time and when a preset limit is reached the radiation source is turned off and the exposure terminated. See for instance U.S. Pat. No. 5,331,166 issued in 1994 to Yamamoto et al. and/or U.S. Pat. No. 5,461,658 issued Oct. 24, 1995 to Joosten for typical exposure control systems used with electronic or direct radiography imaging systems.
A problem with most prior art systems that rely on intensity integration is that the intensity of the radiation incident on the control sensor depends on the relative position of the patient and the sensor. If it is intended that the patient and sensor be so positioned that the sensor receives radiation passing through soft tissue only, placing the patient improperly may result in radiation passing through bone rather than soft tissue prior to impinging on the sensor. In such case, it is obvious that the exposure will no longer be optimal since the sensor will control the duration of the exposure assuming that it is receiving radiation passing through soft tissue rather than bone.
In an effort to minimize this problem, radiation sources have been developed with associated patient placement aids. Typical of such aids are projection systems that project a visual pattern aligned with the radiation beam onto the patient, such as a luminous field and/or a crosshair. The technician places the patient so that the pattern falls into predefined areas of the patient's anatomy. Other systems provide markings on the surface supporting the patient or the imaging panel, indicating the position of the exposure control sensor thereunder, so that the patient may be properly placed to allow for optimum operation of the system.
More complex systems using multiple inputs from a plurality of sensors, or systems analyzing the output of imaging sensors have also been developed in an effort to obtain optimum exposure. U.S. Pat. No. 5,084,911 issued to Sezan et al. exemplifies such a system where proper exposure is calculated by selecting one or more signals from the imaging panel array of X-ray sensors and calculating exposure using the selected signals.
However, none of the prior art systems allow the technician or doctor to determine when viewing a displayed image whether a less than optimal exposure is the result of system failure or poor patient positioning relative to the exposure control sensor or sensors. Such determination is very useful both in calibrating the exposure system and in training radiology technicians in proper patient placement and in diagnosing the cause of incorrect or sub-optimal exposure levels. There is thus still a need for a system that will indicate the relative position of the exposure control sensor or sensors during exposure after the exposure is terminated.